In the manufacture of electronic products, it is generally required that the wiring of the product be verified by a test system. The electronic products which require such wire verification tests include substrates, printed wiring boards, multi-layer ceramic substrates, etc. Although there are a multitude of wire verification tests that can be performed, the more common types of verification tests are the so-called continuity or short-circuit test and the so-called insulation or open-circuit test. Broadly, a continuity test verifies that an electrical connection or path exists between terminals or points of an electronic product or a device under test (DUT); and an insulation test verifies the absence of electrical connection or the existence of device leakage between points or sets of points of a DUT, i.e., an insulation test verifies that points or sets of points are mutually isolated from each other. The terminals or points are typically terminations of a network of the DUT.
Current day test systems commonly utilize solid state circuitry and employ matrix-type switching systems capable of extremely high-speed operation. For example, the switching system may be capable of being connected to over 260,000 terminals or points, thus allowing for the test system to perform on the order of 6,000-8,000 tests per second. Moreover, the test systems typically have random access capabilities which allow for accessing and testing of the various terminals or points located on a DUT in any desired pattern and sequence.
The testing scheme generally utilized by these test systems to verify wiring includes applying electrical stimulus or signals between selected points of a DUT, measuring the electrical resistance between the points, and comparing the measured resistance to a specified limit. Generally, the specified limit is a user defined maximum or minimum resistive quantity which takes into account known resistances of wires, connectors, etc., which is used to determine whether there should be considered a short-circuit or open-circuit existing between points of an electronic product In this regard, a low limit is a resistive quantity that the measured resistance must not exceed in order for there to be considered a short-circuit existing between the points; and a high limit is a resistive quantity that the measured resistance must exceed in order for there to be considered an open-circuit existing between the points. As an example, the low limit may be on the order of 10 ohms, and the high limit may be on the order of 10 Mohms.
A common problem associated with these test systems is the presence of electrical resistance at the contacts or interfaces between the probes of the test system and the terminals of the DUT being probed. Many causes of this contact resistance are unavoidable. For instance, it is required that small diameter probes be utilized in order to contact small, closely spaced apart terminals; and it is further required that relatively poor electrically conducting probes be utilized in order to maintain the required probe hardness to prevent breakage of the probes when a contact force is applied thereto. Inevitably, this contact resistance is included in the measurement of resistance between points of a DUT, thus causing inaccuracies when conducting short-circuit and open-circuit tests. In other words, the resistance that is measured between terminals of a DUT includes the resistance of the network between the terminals, as well as the contact resistance at the interfaces between the probes of the test system and the terminals of the DUT. Unfortunately, there is no practical method of distinguishing between these resistances when testing.
The inaccuracies caused by contact resistance may be the passing of a network that actually should fail, or the failing of a network that actually should pass. One solution proposes compensating for contact resistance when setting the limits. In other words, adding a compensation resistance to the limit settings in order to compensate for contact resistance. In this regard, the compensation resistance would be an approximation of the contact resistance. However, such a solution has proven unsuccessful since contact resistance varies considerably from one set of probes and terminals to another set of probes and terminals. A further problem is that contact resistance may vary from one test to another even if the same set of probes is used to contact the same set of terminals. Measurements have shown that contact resistance can be anywhere between 1 ohm and 35 ohms. Moreover, measuring the contact resistance between each and every probe and terminal is highly impractical, since there can be between 5,000-35,000, or as much as over 260,000, probe/terminal contacts that would require such a resistance measurement for a given electronic product.
Generally, contact resistance has been found to be a greater problem in testing for short-circuits or continuity as compared to testing for open-circuits. Two examples of how contact resistance and compensation resistance can cause inaccurate results is given below:
______________________________________ low limit = 10 ohms compensation resistance = 5 ohms compensated low limit = 15 ohms contact network measured resistance resistance resistance ______________________________________ Example I 9 ohms 8 ohms 17 ohms Example II 2 ohms 12 ohms 14 ohms ______________________________________
Since the resistance of the network of Example I is 8 ohms, which is less than the low limit of 10 ohms, the network of Example I should pass as a short-circuit. However, because of the contact resistance of 9 ohms, the resistance measured by the test system is 17 ohms, which is greater then the compensated low limit of 15 ohms. Therefore, erroneously, the network of Example I will fail the test, i.e., the network will not be regarded as a short-circuit. Similarly, Example II illustrates how compensating for contact resistance can allow a network to pass as a short-circuit when it should actually fail.